Telephone circuits operate on relatively low voltages. Such circuits are sometimes exposed to transient voltages of considerably higher levels than the normal operating voltages. This may be effected by a lightning strike, by momentary contact with a high voltage line, or by other causes. Therefore, it is desirable to provide means for protecting low voltage telephone circuits against such high transient voltages.
It is known to provide plug-in structures for use with telephone terminal blocks to provide connection thereto, including electrical components for protecting against high transient voltages, see for example U.S. Pat. Nos. 4,113,340, 4,116,524 and 4,126,369.